Increased interest in the global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires, along with steering stability and braking performance when traveling on wet road surfaces, and superior safety performance. As a result, by compounding silica in rubber compositions that form tread portions, dynamic visco-elasticity characteristics of the tread rubber such as loss tangent (tan δ) and the like has been improved, heat build-up has been suppressed, rolling resistance reduced, and fuel consumption performance improved, which has led to enhancements in wet performance. However, silica has poor affinity with diene rubber and dispersibility tends to be insufficient. Particularly, when the particle diameter of the silica is small, dispersibility worsens and, as a result, the effects of achieving reduced heat build-up and improving wet performance have not been obtainable.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498 proposes improving the dispersibility of silica by compounding silica in a rubber composition with a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.) and enhancing wet grip performance (tan δ at 0° C.). Additionally, Japanese Unexamined Patent Application Publication No. 2007-321046 proposes a rubber composition comprising a styrene-butadiene copolymer rubber and, per 100 parts by weight thereof, from 80 to 180 parts by weight of a filler comprising not less than 50 parts by weight of a silica, and from 5 to 60 parts by weight of a resin having a softening point of 100 to 150° C.
However, the rolling resistance and wet performance obtained by the rubber compositions proposed in Japanese Unexamined Patent Application Publication No. 2009-091498 and Japanese Unexamined Patent Application Publication No. 2007-321046 are below those demanded by users and there is a need for further improvement in low rolling resistance and wet performance.